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    justify-content 属性

    align-content 属性

    place-content 属性

    justify-content属性是整个内容区域在容器里面的水平位置（左中右）

    align-content属性是整个内容区域的垂直位置（上中下）。
    justify-content: start | end | center | stretch | space-around | space-between | space-evenly;
    align-content: start | end | center | stretch | space-around | space-between | space-evenly;
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